The invention relates to magnetic transducers for reading information bits from a magnetic medium. In particular, the invention relates to an improved method of making tunneling magneto-resistive (TMR) read heads and the improved head.
A magneto-resistive (MR) element exhibits a change in electrical resistance as a function of external magnetic field. Such property allows MR elements to be used as magnetic field sensors, read heads in magnetic storage systems, and magnetic random-access-memories. In storage systems, the read head is typically merged with a writer head. The writer writes encoded information to the magnetic storage medium, which is usually a disk coated with hard magnetic films. In a read mode, a magnetic bit on the disk modulates the resistance of the MR element as the bit passes below the read head. The change in resistance can be detected by passing a sense current through the MR element and measuring the voltage across the MR element. The resultant signal can be used to recover data from the magnetic storage medium. Depending on the structure of a device, the MR effect can fall in to different categories, namely, anomalous MR (AMR), giant MR (GMR), tunneling MR (TMR), and colossal MR (CMR).
Most hard disc read heads currently in production utilize an AMR sensor. The essential structure consists of a stripe of soft magnetic material, usually an alloy of Ni, Fe and/or Co. For areal densities beyond about 10 Gbit/inch2, AMR heads give way to GMR heads due to lack of signal.
The GMR device favored by the data storage industry is the spin valve. It consists of a free layer which rotates with the external field, a Cu spacer, and a pinned layer which has a magnetization fixed along one direction. The electrical resistance of a spin valve is a function of the angle between the magnetization in the free layer and the pinned layer. A sensor is the most resistive when the two layers are magnetized in anti-parallel directions, and is the most conductive when they are parallel. Industry has invested heavily in developing a GMR read head, including some mass production. The technology can possibly work for areal densities up to 100 G bit/inch2, beyond which point the sensitivity again becomes an issue.
One possible solution is to use TMR junctions which can give two to three times more signal. In addition, TMR junctions offer more room for engineering design as the TMR effect is less sensitive to the structure of the element than GMR. In particular, the magneto-resistance is usually not related to the junction area, junction resistance, and film thickness.
A TMR junction is very similar to a spin valve in the sense that it also consists of a free layer, a spacer, and a pinned layer. The magneto-resistance rises from the angular difference between the magnetization in the two magnetic layers in a way entirely analogous to a spin valve. A major difference between TMR junction and spin valve is that the spacer in a TMR junction is made of an insulator, typically aluminum oxide. Moreover, the electrical current is flown perpendicular to the plane of the films as oppose to in the plane for GMR sensors. Consequently, one must attach a top and a bottom electrode to the junction stack in order to measure the electrical property.
CMR effect has so far been limited to cryogenic temperature and/or in extremely high magnetic field up to 10,000 Oe. Industrial applications have therefore been limited.
Regardless of the different types of MR elements, such structure is further shielded by high permeability films, like NiFe, in a read head. In some cases, the active sensor and leads are isolated from the shields by insulator material like metal oxide or nitride. The function of the shields is to protect sensor from the stray magnetic field originating from all magnetic bits on the medium, except the one just underneath the sensor.
Fabrication of a sensor involves several deposition, etching, and photo processes. Typically, an insulator layer is deposited on a ceramic substrate and then polished. A first magnetic shield is deposited and shaped, followed with deposition of a thin insulator layer called first half gap. Then, a series of depositions, etching, milling and lift-off processes are performed to fabricate the active sensor. The sensor structure is then covered with an insulator layer called a second half gap after which follows the deposition of a second shield, also referred to as the shared pole. The writer structure can be built over the second shield. A thick insulator can be deposited to encapsulate the whole structure, sensor and writer, after which the structure can be polished again. Pads are applied to the electrical leads for later wire bonding to an external circuitry. Finally, the wafer can be sliced into bars each carrying an array of sensors. Bars can be lapped to obtain sensor of a desired dimension. During machining process several photo and ion-mill operations are conducted to grove a proper air bearing design used later for slider to fly at a desired altitude on a magnetic medium. Each bar is then diced into individual sliders.
A TMR read head was first disclosed in U.S. Pat. No. 5,390,061 by Hitachi, Ltd.; however, this did not include horizontal bias. An improved design with horizontal bias was disclose by IBM in U.S. Pat. No. 5,729,410. IBM has subsequently improved their design in U.S. Pat. Nos. 5,898,547, 5,898,548, and 5,901,018. Specifically, the IBM patents showed a flux guide design, which is suited for a TMR reader. The design allows the usage of a large area junction. This helps to reduce the junction resistance, which remains the leading obstacle for high density reader. The problem with the IBM design is that the tunnel barrier was made in an ex-situ fashion, i.e. the barrier was exposed to air before the deposition of the top electrode. In practice, such method yields junctions with unacceptably poor quality. What is needed is a TMR head with a quality tunnel barrier.
Accordingly, the present invention provides a method for forming a flux-guide type TMR head with an in-situ tunnel barrier and the resultant structure. The TMR head includes a spin-dependent tunneling (SDT) junction utilizing an aluminum oxide tunnel barrier. The tunnel barrier can be formed to a thickness comparable with a typical Cu spacer layer on a spin valve. With the SDT junction, current is applied perpendicular to the plane of the film. The SDT junctions can have high magneto-resistance up to 40%. The magnetoresistive qualities of a head design incorporating the SDT junction are not directly related to head resistance, head geometry, bias current and film thickness.
According to one aspect of the invention a method for forming a spin tunnel barrier is disclosed wherein the stack is fashioned into a bottom electrode. A junction is defined from a tri-layer portion of the stack. A layer of insulator is deposited over the junction and the photoresist layer used to form the junction and insulator layer is lifted off. An upper electrode can also be deposited, including a flux guide.
The stack fashioned into the bottom electrode can include a pinned layer, a barrier layer and a free layer. The pinned layer can include Ni, Fe, Co, or any alloy of these elements, such as NiFe. The barrier layer is typically formed from an oxide or nitride of Al, Ta or Si, such as, AlOx. The free layer, like the pinned layer can include Ni, Fe, Co, or any alloy of these elements, such as NiFe.
Typically, the junction will be defined with an ion mill or sputter etch. Similar to the barrier layer, the insulator can include an oxide or nitride of Al, Ta or Si, such as, Al2O3. The top electrode layer will typically be formed from Cu.
According to another aspect, the invention discloses a method of creating a spin-tunnel junction head on a sheet film. The method includes depositing a pinned layer and a tunnel barrier. A first free layer can then be deposited onto the tunnel barrier, and a subsequent capping layer formed above the first free layer. A junction can be cut into the structure with an ion mill or sputter etch process. An insulation layer can also be deposited, after which the capping layer and a portion of the first free layer can be removed. The capping layer can be removed with a sputter etch or ion mill process. Finally, a second free layer can be deposited.
In another embodiment, the invention additionally includes depositing a conduction layer and etching the second free layer. In addition, a permanent magnet can be deposited as well as an exchange tab.
In another aspect of the invention a recessed shield can be used to cause media flux to penetrate to a depth approximately equal to the junction height. The recessed shield can allow for a junction that is wider than the width of a reader portion of the head.
In still another embodiment, a spin tunnel junction magnetoresistive head including a pinned layer, a tunnel barrier, a first free layer, a capping layer, a junction, an insulation layer and a second free layer is disclosed. A conduction layer, a permanent magnet portion and an exchange tab can also be included
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Implementations may provide advantages such as facilitating access to support documentation and device drivers. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims.